1. Field of the Invention
The present invention relates to implantable hearing systems for rehabilitation of pure sensorineural hearing losses, or combined conduction and inner ear hearing impairments. In particular, the invention relates to such hearing system in which as implantable sensor delivers an electrical audio signal which is processed by an implanted processor and delivered to an to implantable electromechanical transducer which acts on the middle or inner ear.
2. Description of Related Art
Fully implantable hearing systems for rehabilitation of a pure sensorineural hearing loss, or combined conduction and inner ear hearing impairment, with mechanical stimulation of the damaged ear will soon be available on the market. Examples of these systems are disclosed in the journal HNO 46:844-852, 10-1998, H. P. Zenner et al., "Initial implantations of a completely implantable electronic hearing system in patients with sensorineural hearing loss"; U.S. Pat. Nos. 5,277,694; 5,788,711; 5,814,095; 5,554,096 and U.S. Pat. No. 5,624,376. These hearing systems have basically four function units, specifically a sensor (microphone) which converts the incident airborne sound into an electrical signal, an electronic signal processing and amplification unit, an electromechanical transducer which converts the amplified and preprocessed sensor signals into mechanical vibrations and sends them via suitable coupling mechanisms to the damaged middle and/or inner ear, and an electrical power supply system which supplies these modules. Furthermore, a unit may be provided which supplies electrical recharging energy to the implant, when the implant-side power supply unit contains a rechargeable (secondary) battery, for example as shown in U.S. Pat. No. 5,279,292. A telemetry unit may also be provided with which patient-specific audiological data can be bidirectionally transmitted wirelessly or programmed in the implant and thus permanently stored as disclosed in the journal HNO 46:853-863, 10-1998, H. Leysieffer et al., "A completely implantable hearing system for inner ear hearing handicapped: TICA LZ 3001".
Especially in fully implantable systems is the visibility of the system not an issue. As a result, in addition to the advantages of high sound quality, the open auditory canal and full suitability for everyday use, high future patient acceptance can be assumed. Basically, in these implantable systems, the output signal is a mechanical vibratory stimulus which directly excites the middle ear or inner ear. The coupling of the mechanical excitation which is produced by an electromechanical transducer takes place by direct mechanical connection of the vibrating transducer element to the ossicle chain or an ossicle of the middle ear or to the inner ear, e.g. commonly owned co-pending U.S. patent application Ser. No. 09/042,805 filed Mar. 17, 1998, or by force coupling via an air gap in electromagnetic transducers, for example.
The airborne sound signal is converted into an electrical signal which can be further processed and amplified in an electronic unit and then conditioned, by a special sensor (microphone) which is positioned subcutaneously in fully implantable hearing systems, i.e. under the closed skin. The electrical signal triggers the implanted electromechanical transducer for middle ear or inner ear excitation. The area of the auditory canal, the eardrum itself or the malleus which has fused with the eardrum or other ossicle of the middle ear, has been selected to be acoustically and audiologically advantageous as the implantation site of the sensor. The sensor is basically a mechano-electrical converter with an input signal which is a mechanical vibration which results from the acoustic density wave of the incident airborne sound. These implantable, acoustic or mechano-electrical sensor systems are described in the scientific literature such as the journal HNO 45:816-827, 10-1997, H. Leysieffer et al., "An implantable microphone for electrical hearing implants" and in published U.S. Pat. No. 5,814,095, commonly-owned co-pending U.S. patent application Ser. No. 09/097,710 filed Jun. 16, 1998, U.S. Pat. Nos. 4,729,366, 4,850,962 and published PCT Application No. 98/36711, published PCT Application No. 98/06237, U.S. Pat. No. 5,859,916, published PCT Application Nos. 98/03035, 99/08481, 99/08475, 99/07436, 97/18689.
The function of this sound-converting sensor or microphone in a fully implantable hearing system is to convert the external, incident airborne sound into an electrical signal which can be sent to a subsequent electronic signal processing and amplification unit. Since in a full implant, for reasons of biostability and hygiene, it is fundamentally critical that no artificial and permanent body opening or skin opening be produced by which the sound to be converted can be supplied to a sensor or microphone, there is biologically active tissue between the sensor elements and the external, sound-carrying medium, i.e. air. This tissue can be the skin of the auditory canal for a microphone implanted subcutaneously in the posterior wall of the auditory canal, as discussed in the journal HNO 45:816-827, 10-1997, H. Leysieffer et al., "An implantable microphone for electronical hearing implants", U.S. Pat. No. 5,814,095 and the above-mentioned U.S. application Ser. No. 09/097,710. Alternatively, in the case of direct mechanical sensor coupling to the eardrum or the malleus in the tympanic cavity or in hermetically sealed housings in retroauricular, subcutaneous areas of the mastoid, it can be a bony or cartilaginous structure such as disclosed in U.S. Pat. No. 4,729,336, U.S. Pat. No. 4,850,962, published PCT Application No. 98/36711, published PCT Application No. 98/06237, U.S. Pat. No. 5,859,916, published PCT Application Nos. 98/03035, 99/08481, 99/08475, 99/07436, 97/18689. In these types of sensor or microphone placements, which is always done subcutaneously, the uncertainty of a long-term stable, reliable coupling exists since the placement can be affected by necroses formation, temporary or permanent seromae, tissue regeneration (for example, connective tissue), air pressure changes in hermetically tight pressure converter sensors and other external and internal actions. Even if these influences can be minimized by suitable sensor design, the interindividual anatomy, which can likewise affect the sensor transfer function, remains as a variable factor.
Exact knowledge of this sensor transfer function, i.e. the frequency-dependent acoustic pressure transfer factor, which quantitatively describes the conversion of acoustic pressure into a proportional electrical signal, is of great importance for individual adaptation of the audiological hearing system parameters, for example the frequency-dependent amplification. Intraoperatively, an a priori estimate of sensor function data can be determined using suitable measurement methods, but this data is certainly not identical to those of the healed, postoperative state. In particular, measurements of the acoustic directional characteristic of the sensor in the implanted state are of great interest; these measurements are basically impossible intraoperatively.